The closest prior art to the present invention is believed to be WO03/016684 which is incorporated herein by reference. In the latter patent specification, a summation lever following the movements of two cams is pivotably mounted on a rocker to open a poppet valve by an amount equal to the sum of the lifts occasioned by the separate cams. By varying the phase of the two cams relative to one another, the event duration and the valve lift can be modified and by simultaneously varying the phase of both cams in relation to the engine crankshaft, the valve timing can be modified.
A disadvantage of this earlier proposal is that it requires the cylinder head architecture to be redesigned.